A wireless communication system includes: an access point used the first channel or second channel to transmit the data, wherein the center frequency of the first channel is an initial frequency and the center frequency of the second channel is an target frequency; and a client terminal. When the client terminal receives the data from the first channel, the access point and the client terminal are operated by a first working frequency and second working frequency respectively. Initially, both working frequencies are located at the initial frequency. When the first channel is desired to switch to the second channel, the first working frequency is added an offset frequency. The second working frequency is adjusted and followed so as to keep up with first working frequency. By adding the offset frequency to the working frequency repeatedly, the first and second working frequencies are shifted until both of them arrive at the target frequency.